In the production of certain monomers, ethylenically unsaturated hydrocarbons are oxidized, usually in the presence of a suitable catalyst to form the desired monomer. For example, one method for producing acrylic acid is by vapor-phase oxidation of propylene or acrolein in the presence of a catalyst. Similarly, the methacrylic acid can be produced by vapor-phase oxidation of isobutylene, tertiary butanol, tertiary-butyl methyl ether, methacrolein or isobutyraldehyde. The products which result from these processes are aqueous solutions of monomer which are contaminated with undesirable by-products. The aqueous monomer solutions are then extracted with a suitable solvent to recover the monomer. The organic phase containing the monomer is then stripped of the solvent in a solvent-separation step to obtain the monomer product. The low-boiling impurities are then distilled from the monomer product. Finally, the monomer product is distilled to separate high-boiling impurities.
Some of the impurities in the monomer product include acrolein, methacrolein, propionic acid, acetic acid, acetaldehyde, maleic acid, benzoic acid, terephthalic acid, and toluic acid. These by-products, or impurities, can impart color to the product or can act as polymerization inhibitors. Additional processing steps, usually distillations, are required to reduce or remove these impurities, thereby increasing the cost of manufacturing pure monomer products.
One of the impurities formed as a by-product in the production monomers such as acrylic acid and methacrylic acid is protoanemonin: ##STR2##
The art has long sought an efficient and cost effective method of reducing the levels of protoanemonin in monomer solutions. Three general approaches have emerged: treating the aqueous monomer solution resulting from the vapor-phase oxidation, treating the extracted monomer/solvent mixture, and treating the glacial monomer.
A representative method of reducing by-products by treating an aqueous monomer solution is described in Japanese patent 62-045219. The method disclosed therein requires treating an aqueous acrylic acid solution with bisulfite, such as an alkali metal bisulfite or ammonium bisulfite before performing the extraction. Japanese patent 62-045219 discloses that this method is effective for reducing the levels of acrolein, propionic acid, acetic acid, acetaldehyde, carbon monoxide, carbonic acid gas, maleic acid and aromatic acids such as benzoic acid and terephthalic acid. These references do not disclose or suggest reduction of protanemonin levels.
A representative method of reducing by-products by treating an extracted monomer/solvent mixture, is described in European patent application 102642. The method disclosed therein requires treating an extracted methacrylic acid/solvent mixture with an aqueous bisulfite solution, such as an alkali metal bisulfite or ammonium bisulfite, followed by a separation step. European patent application 102642 discloses that this method is effective for reducing the levels of protoanemonin in methacrylic acid. This reference does not disclose or suggest reduction of protanemonin levels by treating the aqueous monomer solution.
Japanese patent application 61-218556 discloses a method of treating either an extracted acrylic acid/solvent mixture or a glacial acrylic acid to lower the levels of impurities. Japanese patent 64-004505 discloses a method of treating either an extracted methacrylic acid/solvent mixture or a glacial methacrylic acid to lower the levels of impurities. These references disclose that after the addition of bisulfite which is introduced into the aqueous monomer solution, the addition of hydrazine compounds to the extracted monomer/solvent mixture or to the glacial monomer, further reduces the levels of acrolein, propionic acid, acetic acid, formic acid, acetaldehyde, formaldehyde, carbon oxides, maleic acid, furfural, protoanemonin, and aromatic acids such as benzoic acid and terephthalic acid in the monomer product. These references do not disclose or suggest reduction of protanemonin levels by treating the aqueous monomer solution.
Japanese patent 81-41614 discloses a method of reducing the level of protoanemonin in acrylic acid by treating either the aqueous acrylic acid solution resulting from the vapor-phase oxidation, the extracted acrylic acid/solvent mixture, or the glacial acrylic acid. The method disclosed therein requires the addition of 0.5% to 1% by weight of the solution to which it is being added of a nitrous acid salt, nitrogen oxide or nitrobenzene, and a polymerization inhibitor.
U.S. Pat. No. 3,725,208 is directed to a method of treating glacial acrylic acid to reduce the levels of aldehyde impurities. This patent discloses that the addition of sulfuric acid, hydrazine, phenylhydrazine, aniline, monoethanolamine, ethylenediamine or glycine to glacial acrylic acid followed by a distillation results in a reduction in the level of aldehyde impurities in the acrylic acid.
U.S. Pat. No. 3,893,895 is directed to a method of treating glacial 1,2-unsaturated carboxylic acids to reduce the level of carbonyl compounds which are present as impurities. The carbonyl compounds include acrolein formaldehyde, methacrolein, crotonaldehyde, acetaldehyde, hexen-2-al, acetone and furfural. According to the disclosure of the U.S. Pat. No. 3,893,895, the levels of these compounds in the 1,2-unsaturated carboxylic acids are reduced by treating the glacial acid with an amine and distilling the mixture. The amines which are disclosed as being useful are inorganic amines, primary and secondary aliphatic and aromatic amines, such as hydrazine, hydroxylamine, 1,2-ethanolamine, 1,2-ethylenediamine, octyl amine, 1,3-propanolamine, 1,2-propanolamine, octadecyl amine, aniline, p-phenylenediamine, o-phenylenediamine, 1,2-dianilinoethane, alpha naphthyl amine, beta naphthyl amine, p-methyl aniline, o-methyl aniline, N-methyl aniline, semi-carbazide, phenyl hydrazine, and 2,4-dimethyl aniline. This reference does not disclose or suggest reduction of protanemonin levels.